The present invention relates to a drive circuit for an AC-ignition circuit, which includes at least one capacitor and at least one trigger element, which is used for deploying a restraint system in a motor vehicle, in which a bridge circuit having four electronically controllable switches is being provided, of which two switches are connected in series between a supply voltage and the ground and the ignition circuit forms a bridge shunt between the two switches connected in parallel; in the case of a fault occurring in the bridge circuit only those switches are activated which guarantee charge and discharge of the capacitor, in spite of the fault.
A drive circuit is described in German Published Patent Application No. 44 47 174. In systems relating to safety such as restraints (e.g., airbags, seat belt tighteners) in vehicles, the activation of ignition circuits should always be guaranteed, even when the drive circuit is faulty. For this purpose the drive circuit described in the document cited provides circuit variants of transistors in the bridge circuit for various types of fault (e.g., short circuits in the shunt of the bridge circuit to ground or to the supply voltage), which guarantee charging and discharging of the capacitor in spite of the fault. It does not emerge from German Published Patent Application No. 44 47 174 that the drive circuit responds to a defect of one of the electrically controllable switches existing in the bridge circuit in order to charge or discharge the capacitor in the event of such a fault.
Thus, an object of the present invention is to provide a drive circuit, which guarantees activation of the ignition circuit in the greatest possible number of fault occurrences, in particular when one of the electronic switches in the bridge circuit is defective.
The object is achieved in that, in the event of a defect in one of the two switches connected to ground, the capacitor is charged by closing the switch connected to the supply voltage, and connected in series with this defective switch, and by closing the switch connected to ground while belonging to the other pair of switches, and the capacitor is discharged by closing both switches connected to the supply voltage.
The capacitor can also be charged, when there is a defect in one of the switches connected to the supply voltage, by closing the switch connected to ground and connected in series with the defective switch and closing the switch connected to the supply voltage belonging to the other pair of switches, and the capacitor can be discharged by closing both switches connected to ground.
In addition, in the event of a short circuit of one of the two terminals of the ignition circuit to ground, the capacitor can be charged by closing that switch, connected to the supply voltage, which is connected to the terminal of the ignition circuit that is not short circuited. The capacitor is discharged by closing both switches connected to ground.
Moreover, when there is a short circuit of one of the two terminals of the ignition circuit to the supply voltage, the capacitor can be charged by closing that switch connected to ground which is connected to the terminal of the ignition circuit that is not short circuited. The capacitor is discharged by closing both switches connected to ground.
The circuit according to the present invention thus facilitates control of the ignition circuit even in the event of a very large number of possible occurrences of fault so that the restraint can be deployed without impediment in a motor vehicle crash.